1. Field of the Invention
The present invention relates to a plasma CVD interlayer dielectric film formed between wiring conductor layers in a semiconductor device, and a process for forming the same.
2. Description of Related Art
Recently, there have been advances in micro-fabrication of a semiconductor integrated circuits. In particular, the trend is to multilevel interconnections in a logic integrated circuit. If spacing between adjacent metal wiring conductors in the multilevel interconnection becomes small, capacitance between the adjacent metal wiring conductors becomes large, with the result that various disadvantages occur. For example, the speed of an electric signal drops, and cross talk (imparting influence to another signal as noise) occurs.
One countermeasure is to form an interlayer dielectric film of an insulating material having a low dielectric constant. Recently, there has been reported to lower the specific dielectric constant from the order of 4.5 to the order to 2.8 to 4.3, by changing a plasma silicon oxide film (called a xe2x80x9cP-SiO2 filmxe2x80x9d hereinafter) which was used in the prior art and which was formed in a plasma chemical vapor deposition (called a xe2x80x9cP-CVD processxe2x80x9d hereinafter), to a fluorine containing plasma silicon oxide film (called a xe2x80x9cP-SiOF filmxe2x80x9d hereinafter).
The dielectric constant can be lowered by increasing the fluorine concentration in the. P-SiOF film. However, if the fluorine concentration becomes too large, xe2x80x9cresistance to moisturexe2x80x9d (or xe2x80x9cresistance to water absorptionxe2x80x9d) is deteriorated. Therefore, at a fluorine concentration which does not deteriorate the xe2x80x9cresistance to moisturexe2x80x9d, the dielectric constant cannot be so lowered (for example, dielectric constant on the order of 3.3). This is reported by N. HAYASAKA et al, xe2x80x9cFluorine Doped SiO2 for Low Dielectric Constant Films in Sub-Half Micron ULSI Multilevel Interconnectionxe2x80x9d, 1995 International Conference on Solid State Devices and Materials, pages 157-159, the content of which is incorporated by reference in its entirety into this application.
In the case of forming a P-SiOF film in a semiconductor device, planarization is indispensable. In chemical mechanical polishing (called a xe2x80x9cCMPxe2x80x9d) used for planarizing the P-SiOF film, since a polishing liquid is used, insufficient xe2x80x9cresistance to moisturexe2x80x9d becomes a difficult problem. Therefore, when the CMP process is used for planarization, there is no means other than to lower the fluorine concentration thereby to resultantly increase the dielectric constant.
Referring to FIG. 1, there is shown a diagrammatic sectional view of a plasma CVD apparatus for illustrating one example of the process for forming the prior art plasma CVD dielectric film. This example of the prior art process for forming a P-SiOF film (which is one kind of plasma CVD dielectric film), is described in, for example, T. FUKADA et al, xe2x80x9cPreparation of SiOF Films with Low Dielectric Constant by ECR Plasma Chemical Vapor Depositionxe2x80x9d, 1993 International Conference on Solid State Devices and Materials, pages 158-160, the content of which is incorporated by reference in its entirety into this application.
In order to form a P-SiOF film, first, as material gases, O2 gas and Ar gas are supplied through a gas nozzle 17 into a plasma chamber 20, and on the other hand, SiF4 gas is supplied through a gas nozzle 18 into a reaction chamber 19 communicating with the plasma chamber 20. Then, in cooperation of a microwave introduced into the plasma chamber 20 and a magnetic field generated by a magnet coil 21 surrounding the plasma chamber 20, an electron cyclotron resonance (ECR) plasma is created, so that the introduced gases are activated. Thus, a P-SiOF film having excellent step coverage property is formed on a wafer 31 held on an electrostatic chuck 22 supplied with a RF bias voltage from a RF power supply 23.
The film thus formed is constituted of Si (silicon), F (fluorine) and O (oxygen), and the fluorine concentration is controlled by means of the flow rate of the SiF4 gas, namely, the SiF4 gas flow ratio (SiF4/O2). However, in the SiF4 gas, Si and F cannot be controlled independently of each other, and therefore, it is not possible to form a P-SiOF film having a satisfactory low fluorine concentration, and therefore, in an actually formed P-SiOF film, a xe2x80x9cresistance to moisturexe2x80x9d is not sufficient.
As a countermeasure for the above problem, there has been proposed to add SiH4 gas so as to control F independently of Si, thereby to form a P-SiOF film having a relatively fluorine concentration. This is reported by T. FUKADA et al, xe2x80x9cPREPARATION OF SiOF FILMS WITH LOW DIELECTRIC CONSTANT BY ECR PLASMA CVDxe2x80x9d, 1995 DUMIC Conference, Pages 43-49, the content of which is incorporated by reference in its entirety into this application.
However, in this proposed process, it is considered that not only Si, F and O but also H (hydrogen) are included in the film, so that the possibility of formation of Sixe2x80x94OH and Hxe2x80x94OH increases, which act as hygroscopic or moisture absorbing sites, with the result that xe2x80x9cresistance to moisturexe2x80x9d is deteriorated. In other words, it is very difficult to determine an optimum condition which resultantly gives a satisfactory xe2x80x9cresistance to moisturexe2x80x9d.
Referring to FIG. 2, there is shown a diagrammatic sectional view of a plasma CVD apparatus for illustrating a second example of a process for forming a prior art plasma CVD dielectric film. This second example of the prior art process for forming the P-SiOF film is described in N. HAYASAKA et al, xe2x80x9cHigh-Quality and Low Dielectric Constant SiO2 CVD Using High Density Plasmaxe2x80x9d, 1993 Dry Process Symposium, pages 162-168, the content of which is incorporated by reference in its entirety into this application.
In this second prior art process, as shown in FIG. 2, as material gases, CF4 gas and O2 gas are supplied through a gas nozzle 24 into a plasma chamber 26 formed by a quartz tube 15 which is transparent to an electromagnetic wave, and a TEOS (tetraethoxysilane) gas is supplied through a gas nozzle 25 into a reaction chamber 27. Due to the combination of a magnet coil 29 surrounding the plasma chamber 26 and an antenna 28 also surrounding the plasma chamber 26 and driven with a helicon plasma is generated and the gas is activated. Thus, a film is formed on a wafer 31 held on an electrostatic chuck 30.
In the above mentioned second prior art process, no RF bias is applied. An example of applying an RF bias is disclosed by R. KATSUMATA et al, xe2x80x9cImprovement in Hygroscopicity of PE-CVD F-doped SiO2xe2x80x9d, 1995 Dry Process Symposium, pages 269-274, the content of which is incorporated by reference in its entirety into this application. The film formed in this process is constituted of Si, F, H, C (carbon), and O, but the fluorine concentration is controlled by the flow rate of the CF4 gas and the ratio of the CF4 gas to other gases. However, since C and F cannot be controlled independently of each other, a P-SiOF film having a satisfactory xe2x80x9cresistance to moisturexe2x80x9d cannot be obtained.
In the above mentioned prior art dielectric films and the prior art processes for forming the same, because Si and F cannot be controlled independently of each other, as in the SiF4/O2Ar gas supply system, or because C and F cannot be controlled independently of each other, as in the SiH4/O2/Ar/CF4 gas supply system, it is not possible to obtain a dielectric film having not only a low dielectric constant and satisfactory xe2x80x9cresistance to moisturexe2x80x9d but also excellent xe2x80x9cresistance to heatxe2x80x9d. Why this desired dielectric film cannot be obtained will be discussed specifically in the following:
First, carbon has a property of elevating the xe2x80x9cresistance to moisturexe2x80x9d, but if the film contains excess carbon, the xe2x80x9cresistance to heatxe2x80x9d is deteriorated. For example, if the carbon concentration is 1-1022 atoms/cc or more, in the prior art example in which CO2 is used in place of O2, for example, in an example formed by using an SiH4/CO2/Ar/CF4 gas supply system, the obtained film is not resistant to a heat treatment of 400xc2x0 C., and the dielectric constant becomes high.
If the above mentioned control was not conducted, since carbon is short in the gas supply system, when a film is formed by using for example the SiH4/O2/Ar/CF4 gas supply system which doesn""t permit one to control C and F independently of each other, the fluorine concentration becomes higher than the carbon concentration in the obtained film, with the result that the obtained film can have only a deteriorated xe2x80x9cresistance to moisturexe2x80x9d.
Accordingly, it is an object of the present invention to overcome the above mentioned defects of the prior art.
Another object of the present invention is to provide a dielectric film having a low dielectric constant and satisfactory xe2x80x9cresistance to moisture.xe2x80x9d
Still another object of the present invention is to provide a silicon based dielectric film containing fluorine and carbon, which has a low dielectric constant and xe2x80x9cresistance to moisturexe2x80x9d and xe2x80x9cresistance to heatxe2x80x9d enough to give reliability.
A further object of the present invention is to provide a process for forming a silicon based dielectric film containing fluorine and carbon, which has a low dielectric constant and xe2x80x9cresistance to moisturexe2x80x9d and xe2x80x9cresistance to heatxe2x80x9d enough to give reliability.
The above and other objects of the present invention are achieved in accordance with the present invention by a plasma CVD dielectric film containing silicon as a basic material, fluorine in a concentration range of 4.0xc3x971021 atoms/cc to 1.0xc3x971022 atoms/cc, and carbon in a concentration range of 3.0xc3x971019 atoms/cc to 1.0xc3x971021 atoms/cc.
According to another aspect of the present invention, there is provided a process for forming a plasma CVD dielectric film, comprising the step of supplying a material gas composed of a silicon hydride gas, an oxygen gas, a fluorocarbon gas, an argon gas and a carbon oxide gas, into a chamber, and generating a plasma in the chamber to activate the gases, so as to form a plasma CVD dielectric film containing silicon as a basic material, fluorine in a concentration range of 4.0xc3x971021 atoms/cc to 1.0xc3x971022 atoms/cc, and carbon in a concentration range of 3.0xc3x971019 atoms/cc to 1.0xc3x971021 atoms/cc, on a semiconductor substrate located in the chamber.
According to a third aspect of the present invention, there is provided a process for forming a plasma CVD dielectric film, comprising the step of supplying a material gas composed of a silicon fluoride gas, an oxygen gas, an argon gas and a carbon oxide gas, into a chamber, and generating a plasma in the chamber to activate the gases, so as to form a plasma CVD dielectric film containing silicon as a basic material, fluorine in a concentration range of 4.0xc3x971021 atoms/cc to 1.0xc3x971022 atoms/cc, and carbon in a concentration range of 3.0xc3x971019 atoms/cc to 1.0xc3x971021 atoms/cc, on a semiconductor substrate located in the chamber.
In the above mentioned processes, the argon gas can be omitted. In addition, hydrocarbon gas can be substituted for the carbon oxide gas. Furthermore, it is preferred to control the flow rate of the gas containing fluorine and the flow rate of the gas containing carbon, independently of each other.
Specifically, the silicon hydride gas can be one selected from the group consisting of SiH4, Si2H6 (which are generalized by Sin H2n+2), TEOS, SiH2Cl2, or a combination of at least two of the aforesaid gases. The fluorocarbon gas can be one selected from the group consisting of CF4, C2F6, CHF3, C4F8, or a combination of at least two of the aforesaid gases. The silicon fluoride gas can be one selected from the group consisting of SiF4, TEFS (trisethoxyfluorosilane) and a fluorine containing organic silicon compound, or a combination of at least two of the aforesaid gases.
Alternatively, when hydrocarbon gas is used in place of the carbon oxide gas, the hydrocarbon gas can be one selected from the group consisting of C2H2, CH4, C2H6, C3H8. In this case, similarly, the silicon hydride gas can be one selected from the group consisting of SiH4, Si2H6, TEOS, SiH2Cl2, or a combination of at least two of the aforesaid gases. The fluorocarbon gas can be one selected from the group consisting CF4, C2F6, CHF3, C4F8, or a combination of at least two of the aforesaid gases. The silicon fluoride gas can be one selected from the group consisting of SiF4,TEFS and a fluorine containing organic silicon compound, or a combination of at least two of the aforesaid gases.
Furthermore, the silicon hydride gas can be added to the silicon fluoride gas, the oxygen gas, the argon gas and the carbon oxide gas. In this case, the silicon hydride gas can be one selected from the group consisting SiH4, Si2H6, TEOS, SiH2Cl2, or a combination of at least two of the aforesaid gases.
On the other hand, a plasma source for generating the above mentioned plasma is preferred to be a source for generating a high density plasma such as an electron cyclotron re sonance plasma, an inductive cupling be plasma, and a helicon plasma.
The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings.